Abstract

Extensive retrospective analyses of archival medical data collected at The Marine Mammal Center (TMMC) have revealed spatial and temporal patterns of disease prevalence. Two studies were completed and published in 2005 for the 1991-2000 and 1992-2001 periods, which collectively analyze data for California sea lions (Zalophus californianus), Northern elephant seals (Mirounga angustirostris) and Pacific harbor seals (Phoca vitulina), the three most commonly presented species to TMMC^1,2. Additional preliminary data are also now available for the more recent 2006-2009 period.

Along the coast of California, sea lions give birth in June, and wean their pups at 8 months to one year of age. Northern elephant seals give birth in January and wean at four weeks of age. Harbor seals give birth from mid-March to mid-May, depending upon latitude, and wean at four weeks of age. As a result, examining data on a calendar-year basis conveniently reflects the natural history and influences on health of year-classes for each of these three pinniped species. Although there are serious limitations to inferences that can be made using data gleaned from stranded animals, calculation of annual prevalence by etiology can be enlightening ³.

The calendar year 2009 was an exceptional one in the California Current of the North Pacific. In addition to high sea lion pup mortality of the rookeries in June, a record number of animals were presented to the center for rehabilitation attempts or post-mortem research assessments. A total of 1,709 animals presented to the center from 1 January through 31 December of that year. This is an increase of 51% over calendar year 1998 when 1,129 animals were admitted, the previous record high, and 170% over the mean annual number for 1999-2008 (Figure1).

Table 1 is a chart of elephant seal, harbor seal, and sea lion admissions for certain selected etiological categories, by calendar year. It compares annual prevalence for specific disease or syndrome (etiologic diagnoses) over the past four years for common diagnoses at the Center. Animals frequently present with multiple medical problems spanning more than one etiologic category or diagnosis. For example, numerous yearling sea lions present in poor body condition with abscessed lymph nodes. For the purposes of the summary data below for the 2006-2009 period, these animals are included in the statistics for both categories, malnutrition and abscess. This differs from the methodology used in the two prior decade-long studies mentioned above, which assign a primary cause of admission to each animal.

The preliminary statistics for 2008 and 2009 set forth below show a dramatic surge in admissions of animals exhibiting malnutrition, parafilaroides infestation, perforating ulcers, abscesses, and domoic acid intoxication, particularly for California sea lions. Further analysis is required, but it is interesting to note that one of the aforementioned studies observed that strandings due to malnutrition for California sea lions were highest in El Nino years ², and 2009 is again an El Nino year, according to analysis performed by NOAA.⁴ These data and methods also illustrate some of the challenges of inferential biostatistics in clinical medicine.

Figure 1 Annual animal admissions to TMMC by calendar year.

Table 1. Annual prevalence by common etiology for 2005-2009.

Source: TMMC unpublished data
* Individual animals may exhibit more than one condition, so the sum of individual etiological counts does not equal total admissions.
** Selected species only. Does not include fur seals, for example.

Acknowledgements

We gratefully acknowledge the contributions of the many veterinary students, residents, and graduate veterinarians who volunteered time this year to assist with animal care. In addition we thank the Veterinary Technicians, Research Associates and cadre of over 800 volunteers at TMMC for their tireless efforts at improving the quality of life for these animals.

References

1.  Colegrove K, Greig DJ, Gulland FMD. Causes of live strandings of northern elephant seals (Mirounga angustirostris) and Pacific harbor seals (Phoca vitulina) along the central California coast, 1992-2001. Aquatic Mammals 2005, 31(1) 1-10.

2.  Greig DJ, Gulland FMD, Kreuder C. A decade of live California sea lion (Zalophus californianus) strandings along the central California coast: causes and trends, 1991-2000. Aquatic Mammals 2005, 31(1) 11-22.

3.  Gulland FMD, Hall AJ. Is marine mammal health deteriorating? Trends in the global reporting of marine mammal disease. EcoHealth 2007, 4, 135-150.

4.  http://www.elnino.noaa.gov/index.html